Technical Field
The present invention relates to a shift control method and system for a hybrid vehicle, and more particularly, to a shift control method and system for a hybrid vehicle, that increases the recovery rate of regenerative energy, improves the fuel efficiency, and minimizes the loss of the fuel efficiency upon reaccelerating after braking.
Background Art
A hybrid vehicle using an engine and a motor as driving sources is an eco-friendly vehicle which reduces exhaust gases and improves fuel efficiency. As illustrated in FIG.1, in addition to an engine 1 and a drive motor 3 operating as driving sources for driving of the vehicle, the hybrid vehicle includes an engine clutch 2 interposed between the engine 1 and the drive motor 3, a transmission 4 connected to the output side of the drive motor 3, an inverter 5 configured to drive the drive motor 3, and a battery 6 operating as a power source of the drive motor 3 and connected to the drive motor 3 to be charged and discharged through the inverter 5.
Reference number 7 in FIG.1 illustrates a Hybrid Starter and Generator (HSG) power-transmittably connected to the engine 1 to start the engine may be started or to generate electricity by the driving power of the engine. The engine clutch 2 is configured to selectively engage or disengage power transmission between the engine 1 and the drive motor 3 through hydraulic coupling or decoupling, and the inverter 5 is configured to convert a direct current of the battery 6 into a three-p alternating current to apply to the drive motor 3.
Additionally, the transmission 4 is connected to the power output side of the drive motor 3, and is configured to shift and deliver driving power of the engine and the motor to a drive shaft. Such hybrid vehicles either drive in Electric Vehicle (EV) mode, which is purely an electric vehicle mode using the driving power of the drive motor 3, or in Hybrid Electric Vehicle (HEV) mode which uses both driving powers of the engine 1 and the drive motor 3.
Furthermore, during the braking or coasting by inertia, Regenerative Braking (RB) is performed in which inertial energy of a vehicle is recovered by the electricity generating action of the motor to charge the battery (motor charging). A hybrid vehicle is also mounted with a hybrid controller (Hybrid Control Unit (HCU)) as an uppermost controller configured to operate the vehicle, and includes a variety of controllers designed for various devices on the vehicle.
Examples of such controllers include an engine controller (Engine Control Unit (ECU)) configured to operate the engine, a motor controller (Motor Control Unit (MCU)) configured to operate the motor, a transmission controller (Transmission Control Unit (TCU)) configured to operate the transmission, a Battery Management System (BMS) for configured to collect data regarding the state of the battery to use for executing charging and discharging of the battery or to provide to other controllers, and a brake controller configured to execute braking of the vehicle.
The HCU and other controllers exchange data via Controller Area Network (CAN) communication to perform cooperative control. An upper controller is configured to collect various data from lower controllers and transmit control commands to the lower controllers. A hybrid vehicle is also equipped with an Electric Oil Pump (EOP) configured to feed (e.g., supply) hydraulic fluid required for the driving of the engine clutch and the transmission, and a pump controller configured to operate the EOP. The pump controller is disposed to exchange data with an upper controller, i.e., TCU via CAN communication and to operate the EOP based on the control commands applied by the TCU.
Brief description of major functions of controllers involving in regenerative braking in a typical hybrid vehicle is that the hybrid controller is configured to determine regenerative braking command by considering regeneration prohibition conditions and is configured to estimate regenerative braking performance based on the states of the drive motor and the transmission. The brake controller is configured to calculate a required total braking power by the driver's demand (brake pedal operation) and divide between regenerative braking and friction braking to meet the required total braking power by referring to regenerative braking performance transmitted from the hybrid controller.
The brake controller is configured to transmit, to the HCU, the data regarding the regenerative braking allowance obtained as a result of the distribution to be used for the determination of regenerative braking commands, and execute the control on brakes (frictional braking device) to generate a distributed frictional braking force. The MCU is configured to operate the drive motor through the inverter following the regenerative braking command from the hybrid controller, and transmit data regarding motor output torque and others to the hybrid controller to allow the hybrid controller to estimate regenerative braking performance.
The TCU is configured to execute shift control over multi-stage transmission such as Automatic Transmission (AT) and Dual Clutch Transmission (DCT), and transmit data regarding transmission state and others to the hybrid controller to allow the HCU to estimate regenerative braking performance and determine regenerative braking command.
For a hybrid vehicle mounted with a multi-stage transmission, a torque intervention control is performed to reduce a charging torque (e.g., generation torque or regenerative braking torque) during the shift process. Additionally, for a hybrid vehicle mounted with a multi-stage transmission, a regenerative energy difference occurs by shift stage and shift control, where the absolute value of drive motor torque (torque here means motor charging torque which is negative value) reduces by the torque intervention during the transmission process and so does regenerative power. Accordingly, when continuous sequential shifts occur during the braking, motor charging torque is reduced at each transmission shift, thus causing a significant reduction of the total regenerative power.
When further elaborated on the above, FIG. 2 is an exemplary view illustrating a motor speed and a motor torque during a shift process of regenerative braking. The figure shows an example of shift of 5→4→3 stages. When the transmission shifts to low stages during regenerative braking in a related art, a one-stage shift lower is sequentially performed as the speed of the vehicle decreases. Accordingly, a torque intervention occurs at each transmission shift to reduce the absolute value of motor torque (which is a charging torque and has a negative value) which interrupts increase of transmission input speed (e.g., motor speed).
Thus, the charging torque is reduced by the torque intervention, and further loss follows with further transmission events. In a situation of frequent transmission shifts, a loss in recovered energy occurs during regenerative braking since charging power (e.g., regenerative power) decreases as charging torque reduces by torque intervention. For a vehicle driven by the electric oil pump among hybrid vehicles mounted with an automatic transmission, a pump drive loss occurs by a maximum line pressure control at each transmission shift. In other words, when an oil pump of electric motor type is driven for transmission shift, a decrease in fuel efficiency occurs due to an increase of operation energy and power consumption (see FIG. 2) for the EOP since a maximum line pressure control is necessary each time of transmission shift.
Accordingly, it is advantageous in fuel efficiency to perform a skip shift by more than two stages by which to avoid consecutive one-stage transmission shift and to reduce frequency of transmission shifts. However, the skip shift has disadvantage in terms of Shift Quality (SQ) and reacceleration fuel efficiency when the vehicle restarts by minimal accelerator opening degree after braking which may cause loss in fuel efficiency since the engine runs at a substantially high revolutions per minute (rpm) for low shift stage when the vehicle accelerates again.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.